An image display device of a head-mounted type or a face-mounted type, which includes an optical element (e.g., a transparent prism) that has an incident surface, a plurality of reflection surfaces and an output surface; and provides a display image from a display device as a virtual image to an observer, is a so-called HMD and generally known. As such an HMD, recently, an HMD which is used for a display of a so-called wearable computer is studied. Moreover, various HMDs of a see-through type, which allow an observer to observe a virtual image of a display image from a display device and an external image at the same time, are recently proposed.
As HMDs of the see-through type, there are HMDs disclosed in patent documents 1 to 3, for example. All of the HMDs in the patent documents 1 to 3 have a structure which guides image light from a display element (LCD) and external light to an observer's pupil via a prism. Especially, the HMDs in the patent documents 2 and 3 have a structure which reflects the image light two times in the prism and guides the image light to the observer's pupil.